As is well known, a pyroelectric camera is basically a television camera that is capable of producing an infrared television picture. A pyroelectric vidicon tube is utilized as the input of the pyroelectric camera to sense a thermal image of the target by receiving thermal energy emitted by the target and provides an electrical output signal indicative thereof, which electrical signal output is then processed by associated electronic circuitry to provide a composite video output signal.
In order to reduce the readout lag and to be able to read out the positive and negative information from the target of a pyroelectric vidicon tube, it is required that the tube be operated in a mode so as to establish a pedestal component around which the signal information from the vidicon tube is centered. The pedestal component is normally generated either by secondary emission (by means of pulsing the thermionic cathode and beam control grid negative during flyback) or is generated by means of ionizing hydrogen (H.sub.2) gas (during flyback of the reading beam). While either method may be employed in pyroelectric camera systems, both approaches have been found to suffer from stability problems encountered in the amplitude of the generated pedestal level and the effects it has on the output signal and image quality obtained.
A change in pedestal amplitude can cause severe flicker, which flicker occurs at the field rate of the system if analog signal processing is employed, and pedestal level changes can also lead to signal processing amplifier saturation which renders the system useless.
The pedestal level in a pyroelectric camera is influenced by several factors including the cathode pulse amplitude, the beam control grid (G.sub.1) pulse amplitude, and the thermionic cathode. The cathode pulse and the G.sub.1 pulse amplitudes are circuit parameters, and the thermionic cathode characteristic is controlled in manufacture of the camera tube and changes over the life of the camera tube.
If ionization is employed, the gas pressure will greatly influence the pedestal level, and, again, the characteristic of the gas reservoir and the thermionic cathode will change with tube life.
Different control systems and methods have been proposed to stabilize the pedestal level of camera tubes either by monitoring the grid current in the tube and by using its magnitude to control the pedestal level, or by monitoring the grid current and using its magnitude to adjust the hydrogen pressure which then controls the pedestal level.
It has been found, however, that at least some known control systems and methods are difficult to implement and/or have been found to be only partially effective. It has also heretofore been found that while the effect of pedestal level fluctuations might be corrected within limited levels, such correction, if achieved, normally adversely affects available dynamic range.
In addition, at least some known control systems and methods do not provide adequate compensation for the undesirable pedestal level shift generated by the temperature difference between the shutter temperature and the average scene temperature, which temperature difference manifests itself as a DC-pedestal shift and this can also cause flicker, limited dynamic range, and/or possibly amplifier saturation and loss of the image.
With respect to prior art patents, our U.S. Pat. No. 4,481,535 is directed to a signal processing system for a pyroelectric camera that includes separate DC-restorer and automatic gain control (AGC) circuits.